Marine controlled source electromagnetic (CSEM) surveying is a geophysical surveying technique that uses electromagnetic (EM) energy to identify hydrocarbon reserves in formations below the bottom of a body of water such as a lake or the ocean. In a typical marine CSEM survey, an EM source and a number of EM receivers are located at or near the bottom of a body of water. The EM source is typically towed over an area of interest in the Earth's subsurface, and the receivers are disposed on the water bottom over the area of interest to obtain signals related to the distribution of electrical resistivity in the subsurface area of interest. Such surveying is performed for a range of EM source and EM receiver positions. The EM source emits either or both a time varying electric field and a time varying magnetic field, which propagate outwardly into the overlying seawater and downwardly into the formations below the water bottom. The receivers most commonly used detect and record the induced electric field at or near the water bottom. The time varying EM field may be induced by passing electric current through an antenna. The electric current may be continuous wave and have one or more discrete frequencies. Such current passing through an antenna is used for what is referred to as “frequency domain CSEM” surveying. It is also known in the art to apply direct current to an antenna, and produce transient EM fields by switching the current. Such switching may include, for example, switching on, switching off, inverting polarity and inverting polarity after a switch on or switch off event. Such switching may be equally time spaced or may be in a time series known as a “pseudo random binary sequence.” Such switched current is used to conduct what is referred to as a “transient CSEM” survey. One type of such survey is provided under the service mark MTEM, which is a service mark of an affiliate of the assignee of the present invention.
The EM energy is rapidly attenuated in the conductive seawater, but in less conductive subsurface formations is attenuated less and propagates more efficiently. If the frequency of the EM energy is low enough, the EM energy can propagate deep into the subsurface formations. Energy “leaks” from resistive subsurface layers, e.g., a hydrocarbon-filled reservoir, back to the water bottom. When the source-receiver spacing (“offset”) is comparable to or greater than the depth of burial of the resistive layer (the depth below the water bottom) the energy reflected from the resistive layer will dominate over the transmitted energy. CSEM surveying uses the large resistivity contrast between highly resistive hydrocarbons and conductive aqueous saline fluids disposed in permeable subsurface formations to assist in identifying hydrocarbon reservoirs in the subsurface.
FIG. 1 shows a typical marine CSEM surveying system, as illustrated in U.S. Pat. No. 7,191,063 issued to Tompkins. In the arrangement shown in the '063 patent, the subsurface layers of interest include an overburden layer 8, an under-burden layer 9, and a hydrocarbon reservoir 12. A surface vessel 14 moves on the surface 2 of a body of water 4. A submersible vehicle 19 carrying an EM source 22 in the form of a horizontal electric dipole (HED) transmitter 22 is attached to the surface vessel 14 by an umbilical cable 16. One or more remote receivers 25 are located on the seafloor 6. Each of the receivers 25 includes an instrument package 26, a detector 24, a flotation device 28, and a ballast weight (not shown). The detector 24 comprises three orthogonal electric dipole detectors and three orthogonal magnetic field detectors. The electric dipole detectors are sensitive to components of the electric fields induced by the HED transmitter 22 in the vicinity of the receiver 25 and produce corresponding electric field detector signals. The magnetic field detectors are sensitive to components of the magnetic fields induced by the HED transmitter 22 in the vicinity of the receiver 25 and produce corresponding magnetic field detector signals. The instrument package 26 records the detector signals. Recording of data requires complex systems that have to be deployed and positioned on the seabed and record data autonomously when positioned on the seabed. To cover large areas with a dense receiver spacing may be impractical.
FIG. 2A is a schematic of a marine CSEM surveying system, as illustrated in International Publication No. WO 02/14906. The system disclosed in the '906 publication includes a vessel 31 towing a cable (or streamer) 32 just above the seabed 33. The cable 32 carries a transmitter dipole antenna 34 and several receiver dipoles 35. The transmitter dipole antenna 34 is controlled from the vessel 31 via the cable 32, and the responses detected by the receiver dipoles 35 are relayed back to the vessel 31 in real time via the cable 32. The WO 02/14906 publication also shows an arrangement, as illustrated in FIG. 2B herein, in which the vessel 31 tows three parallel cables 41, 42, 43, each carrying a series of receivers 45, 46, 47. The spacing between the receivers 45, 46, 47 is achieved by means of a spar 44. A transmitter 48 is located on the cable 42. The transmitter 48 has two dipole antennae arranged mutually at right angles. Each receiver also comprises two dipoles mutually at right angles. Measurements are taken with the transmitter and receiver both inline and parallel, and the two sets of measurements are compared. A characteristic difference in values indicates a highly resistive layer located beneath highly conductive layer.